3,534 research outputs found
Gravity as Backreaction
Quadratic theory of gravity is a complicated constraint system. We
investigate some consequences of treating quadratic terms perturbatively
(higher derivative version of backreaction effects). This approach is shown to
overcome some well known problems associated with higher derivative theories,
i.e., the physical gravitational degree of freedom remains unchanged from those
of Einstein gravity.
Using such an interpretation of gravity, we investigate a
classical and Wheeler DeWitt evolution of gravity for a
particular sign of , corresponding to non- tachyon case. Matter is
described by a phenomenological . It is concluded that
both the Friedmann potential () and the
Wheeler DeWitt potential () develop repulsive barriers near for
(i.e., ). The interpretations is clear. Repulsive
barrier in implies that a contracting FRW universe ()
will bounce to an expansion phase without a total gravitational collapse.
Repulsive barrier in means that is a classically forbidden
region. Therefore, probability of finding a universe with the big bang
singularity ( ) is exponentially suppressed.Comment: Accepted for publication in Phy. Rev. D.,18 pages, 6 figures, Latex
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Probing the inter-layer exciton physics in a MoS/MoSe/MoS van der Waals heterostructure
Stacking atomic monolayers of semiconducting transition metal dichalcogenides
(TMDs) has emerged as an effective way to engineer their properties. In
principle, the staggered band alignment of TMD heterostructures should result
in the formation of inter-layer excitons with long lifetimes and robust valley
polarization. However, these features have been observed simultaneously only in
MoSe/WSe heterostructures. Here we report on the observation of long
lived inter-layer exciton emission in a MoS/MoSe/MoS trilayer van
der Waals heterostructure. The inter-layer nature of the observed transition is
confirmed by photoluminescence spectroscopy, as well as by analyzing the
temporal, excitation power and temperature dependence of the inter-layer
emission peak. The observed complex photoluminescence dynamics suggests the
presence of quasi-degenerate momentum-direct and momentum-indirect bandgaps. We
show that circularly polarized optical pumping results in long lived valley
polarization of inter-layer exciton. Intriguingly, the inter-layer exciton
photoluminescence has helicity opposite to the excitation. Our results show
that through a careful choice of the TMDs forming the van der Waals
heterostructure it is possible to control the circular polarization of the
inter-layer exciton emission.Comment: 19 pages, 3 figures. Just accepted for publication in Nano Letters
(http://pubs.acs.org/doi/10.1021/acs.nanolett.7b03184
Characterization of aggregate behaviors of torrefied biomass as a function of reaction severity
Several studies have shown that torrefaction can improve various characteristics of biomass, including grindability, flowability, and energy density, at least at the microscopic level. Furthermore, the improvements are often represented as a monotonic function of the torrefaction severity. However, the existing literature is less clear on whether or not these improvements persist at the aggregate level. This paper demonstrates that, at the aggregate level, using differently torrefied biomass in an experimental cookstove as a case study, the relationship between the improvements and torrefaction severity tells a much more complex story than a simple, monotonic correlation. Notably, by defining and measuring various cookstove performance characteristics ranging from stove temperature, effective heat output, and emission profiles, and how these characteristics vary with the severity of torrefied fuel, we conclude that, contrary to the conventional wisdom, more severe torrefaction in many cases does not always lead to more improved fuel characteristics
Correlation between hormone dependency and the regulation of epidermal growth factor receptor by tumor promoters in human mammary carcinoma cells.
Investigation of laser dynamics, modulation and control by means of intra-cavity time varying perturbation
The generation of tunable visible, infrared, and ultraviolet light is examined, along with the control of this light by means of novel mode-locking and modulation techniques. Transient mode-locking of the Nd:YAG laser and generation of short tunable pulses in the visible and the alkali metal inert gas excimer laser systems were investigated. Techniques for frequency conversion of high power and high energy laser radiation are discussed, along with high average power blue and UV laser light sources
Dependence of the flux creep activation energy on current density and magnetic field for MgB2 superconductor
Systematic ac susceptibility measurements have been performed on a MgB
bulk sample. We demonstrate that the flux creep activation energy is a
nonlinear function of the current density , indicating a
nonlogarithmic relaxation of the current density in this material. The
dependence of the activation energy on the magnetic field is determined to be a
power law , showing a steep decline in the activation
energy with the magnetic field, which accounts for the steep drop in the
critical current density with magnetic field that is observed in MgB. The
irreversibility field is also found to be rather low, therefore, the pinning
properties of this new material will need to be enhanced for practical
applications.Comment: 11 pages, 6 figures, Revtex forma
Can Modal Skepticism Defeat Humean Skepticism?
My topic is moderate modal skepticism in the spirit of Peter van Inwagen. Here understood, this is a conservative version of modal empiricism that severely limits the extent to which an ordinary agent can reasonably believe “exotic” possibility claims. I offer a novel argument in support of this brand of skepticism: modal skepticism grounds an attractive (and novel) reply to Humean skepticism. Thus, I propose that modal skepticism be accepted on the basis of its theoretical utility as a tool for dissolving philosophical paradox
Topological Constraints at the Theta Point: Closed Loops at Two Loops
We map the problem of self-avoiding random walks in a Theta solvent with a
chemical potential for writhe to the three-dimensional symmetric
U(N)-Chern-Simons theory as N goes to 0. We find a new scaling regime of
topologically constrained polymers, with critical exponents that depend on the
chemical potential for writhe, which gives way to a fluctuation-induced
first-order transition.Comment: 5 pages, RevTeX, typo
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